This invention relates to a quartz clock having a frequency divider integrated circuit with an adjustable and programmable dividing factor and more particularly to the problem of replacing, in electronic quartz clocks with integrated circuits containing a crystal oscillator and an asynchronous frequency divider, the series trimmer for frequency fine adjustment by making the division ratio between crystal-oscillator frequency and frequency-divider-output frequency adjustable in steps within predetermined limits. The integrated circuits of such quartz clocks are preferably complementary insulated-gate field-effect-transistor circuits, i.e., so-called C-MOS circuits.
Several proposals have been made to solve this problem, as disclosed in German Published applications Nos. 1,946,166, 2,211,441, 2,219,493, 2,233,800, 2,241,514 and 2,250,389 as well as Swiss Pat. No. 534,913. It is a common feature of all solutions of the known prior art except of the first one that the normally m-stage frequency divider must have additional divider stages which, together with a programmable memory circuit and a comparision circuit, influence the m stages of the frequency divider as desired.
In the known prior art this is done in various ways. In the arrrangement disclosed in German Published application No. 2,241,514, for example, the oscillator frequency is chosen to be always lower than the frequency divider's input frequency corresponding to an m-stage binary frequency division, and additional pulses are added at the output or at individual stages of the frequency divider so that the frequency divider's output frequency equals the divided nominal frequency.
In another known solution, namely that disclosed in German Published application No. 2,233,800, the actual oscillator frequency is also chosen to be lower than the nominal frequency, and by intermittently short-circuiting individual frequency-divider stages, the frequency divider is caused to count faster and, thus, the output frequency is approximated to the divided nominal frequency. The duration of the short circuit of individual frequency divider stages is determined by an additional counter, i.e., again by additional frequency-divider stages.
Arrangements disclosed in the other German Published applications mentioned above (except German Published application No. 1,946,166) also influence individual frequency-divider stages in response to signals from comparison or memory circuits by cutting off individual divider stages during certain counting times, which comparison or memory circuits, in turn, are controlled by additional frequency divider stages.
As the aforementioned known solutions to the problem described require at least additional divider stages and other additional circuits large additional circuitry must be provided which, in principle, is monolithically integrable but increases energy consumption to such an extent that such circuits are disadvantageous, particularly for wristwatches.
The above-mentioned German Published application no 1,946,166 discloses another solution to the known no. which requires only one m-stage frequency divider as is used in oscillator circuits with a series trimmer. In that arrangement, the actual oscillator frequency must be higher than the nominal frequency, and at the input of the frequency divider or of a frequency-divider stage near the input, pulses are suppressed intermittently by means of a blanking circuit, with the blanking time being determined by the pulse duration of the output signal of a monostable or bistable multivibrator to which the output signal of the frequency divider is fed as the input signal.
That multivibrator is designed so that the duration of the output pulses can be adjusted from outside by means of an adjustable resistor. Furthermore, the multivibrator is proportioned so as to be able to compensate for changes in oscillator frequency which are due to aging and temperature variations.
It is obvious that the accuracy of this known solution depends on the accuracy of the duration of the multivibrator's output pulses. However, since this multivibrator can be realized only with components having the usual tolerances if the expense is to be kept within reasonable limits, the blanking time is both temperature, age, and supply-voltage-dependent. Furthermore, accuracy is dependent on the accuracy of adjustment of the external adjusting element, which may be an adjustable resistor, for example, so that another factor of uncertainty is created.
Finally, in this known arrangement, the input pulses and the output pulses of the frequency divider are in phase only if the frequency divider is a synchronous counting circuit, while no such phase relationship exists when asynchronous counters are used as frequency dividers. The latter case is generally preferred for frequency dividers in clocks.